Just one more point of view from the employer. Each position is expected to bring in a certain amount of money each year. If your spot is slotted to bring in 1,000,000 each year and you earn the company 950,000 you will not be getting a raise (most likely) On the other hand if you earn the company 1,250,000 a year you should without a doubt you should be given a raise that reflects this. I am not saying that money is the only reason for a raise but it is a large part.
I think this is very important point when getting raises. I work in quality assurance and most people think the department is overhead and doesnt bring in money. Multiple times I was able to take the operation metrics and show that certain projects we implemented saved the company money. That was the most effective way I was able to show how effective I was.